In the particular field of motor vehicles, there can be a need to identify various points on the bodywork of a motor vehicle that has suffered an accident or that is in the process of being assembled, in order to verify its shape by identifying predetermined points of said bodywork by means of a three-dimensional measuring device associated with a reference structure, commonly referred to as a “slab”. Similarly, with a machining robot, it is advantageous to be able easily to identify the points of the motor vehicle bodywork that require special machining, such as milling or drilling.
In this content, reference can be made to patent documents FR-A-2 750 546 and FR-A-2 764 992 in the name of the Applicant.
Proposals have also been made for position-identifying systems using an emitter module carrying a source of light radiation mounted on a stand that can pivot about two distinct axes that are not mutually parallel, associated with two angular position sensors for sensing the rotary movement corresponding to the source relative to the stand, a plurality of targets being disposed around the emitter module in locations that are defined relative to a fixed frame of reference, as described in patent document WO-A-95/35479. Reference can also be made to patent document U.S. Pat. No. 6,611,346 (US 2001/0024283 A1) in the name of the Applicant, describing a position-identifying system using interactive bushings that are integrated in the support wall.
The above systems are entirely satisfactory in terms of accuracy, but they remain very constricting when the machining or measurement machine needs to be moved frequently relative to the article being measured, in particular motor vehicle bodywork.
On each occasion it is necessary to restart the position-identifying process in order to know where the machine is located in the frame of reference of the workpiece or where the workpiece is located in the frame of reference of the machine.
In order to complete the technological background, mention can be made of patent documents U.S. Pat. No. 4,961,267 and EP-A-0 398 073 which illustrate systems for identifying position in two dimensions, the first system also not being self-contained but being associated permanently with a supporting turntable, and the second system using a rod-and-cylinder type sliding assembly. Mention can also be made of patent document FR-A-2 776 373 in the name of the Applicant which discloses another system having a carriage that slides on a rail mounted to pivot about at least one axis.